As electronic devices operate at high speeds or high frequency, solid electrolytic capacitors used for power supply lines for CPUs are demanded to have large capacitance and low impedance in order to improve noise rejection and transient response over a broad bandwidth from low frequencies to high frequencies, such as 1 MHz.
FIG. 23 is a top view of conventional capacitor element 341. FIG. 24 is a bottom view of conventional solid electrolytic capacitor 701 including capacitor element 341. FIG. 25 is a side cross-sectional view of solid electrolytic capacitor 701 at line 25-25 shown in FIG. 27.
Each capacitor element 341 has a rectangular shape, and includes a valve metal plate and separator 350 that divides the valve metal plate into positive electrode 342 and negative electrode 343. Negative electrode 343 includes a dielectric oxide layer provided on the valve metal plate, a solid electrolyte layer made of conductive polymer provided on the dielectric oxide layer, and a negative electrode layer provided on the solid electrolyte layer.
Negative electrodes 343 of plural capacitor elements 341 are stacked such that positive electrodes 342 extend alternately in opposite directions from negative electrodes 343. Positive electrodes 342 of capacitor elements 341 are stacked and welded to positive flat portion 368 of positive terminal 344. Negative electrodes 343 are bonded to negative flat portion 349 of negative terminal 345 with a conductive adhesive. Positive terminals 344 and negative terminal 345 are located on a lower surface of stacked capacitor elements 341.
Capacitor elements 341 are stacked such that positive electrodes 342 extend alternately in opposite directions from negative electrodes 343. This arrangement allows currents to flow through capacitor elements 341 adjacent to each other in directions opposite to each other, and cancels magnetic fields produced by the currents, thus providing capacitor 701 with a small equivalent series inductance (ESL).
Stacked capacitor elements 341 are covered with package resin 346 by molding, such that positive terminal 344 and negative terminal 345 are exposed closely to each other at a lower surface, mounting surface 347 of package resin 346. This structure reduces a length of a path of a current flowing between capacitor elements 341 and lands of a circuit board, accordingly reducing an equivalent series resistance (ESR) and the ESL, impedance of capacitor 701.
In solid electrolytic capacitor 701, side surfaces of negative electrodes 343 of stacked capacitor elements 341 are bonded to a negative electrode coupler provided along the side surfaces of negative electrode 343 with conductive adhesive agent.
In conventional solid electrolytic capacitor 701, capacitor elements 341 may be displaced when capacitor elements 341 are mounted to positive terminals 344 and negative terminal 345, or when stacked capacitor elements 341 are pressurized to be bonded. In this case, positive electrodes 342 of capacitor elements 341 can be connected electrically with negative terminal 345, and prevent positive terminal 344 and negative terminal 345 from being located close to each other, accordingly preventing the impedance from being small.
In conventional solid electrolytic capacitor 701, during a molding process of external resin 346, timing when melted material of external resin 346 flows inside gap 348 between positive electrodes 342 of capacitor elements 341 is different from timing when the melted material flows outside gap 348. This may cause the melted material of external resin 346 to deform positive electrodes 342 to deform by molten external resin 346, and may damage a portion of the dielectric oxide layer or the solid electrolyte layer of negative electrode 343 facing positive electrode 342, thereby increasing a leakage current.
In solid electrolytic capacitor 701, the distance between capacitor elements 341 and mounting surface 347 is small, and accordingly, a portion of external resin 348 that covers a surface of negative flat portion 349 facing mounting surface 347 is thin. The area of negative flat portion 349 covered with external resin 348 is large in order to provide a large capacitance. This structure may cause external resin 348 to be removed from a surface of negative flat portion 349 facing mounting surface 347 due to a rapid temperature change during soldering in a reflow process, thus producing a crack in the resin. This crack may allow oxygen to pass easily through it from an outside of external resin 348, hence increasing an electric resistance of the solid electrolyte layer made of conductive polymer and the ESR, impedance, of the solid electrolytic capacitor.